Multilayer ceramic circuit boards have been in use for several decades for high performance systems such as main frame computers. They are made by preparing a green tape composition from ceramic and glass powders that are mixed with an organic binder and cast. Via holes are punched in the formed tape and wiring patterns of conductive metals are screen printed onto the tapes. Conductive metal inks are also screen printed into the via holes to provide electrical interconnection between the circuit patterns of the various layers. The green tapes are stacked in proper alignment and pressed together so that the vias and wirings contact each other. The multilayer stack is then fired to burn off the polymeric binder and other organic materials, and to sinter the metal patterns and the ceramic layers. Thus ceramic substrates having several layers of interconnected integrated circuits are formed.
The choice of ceramic material determines the type of conductive metal that can be used to make the metal patterns. Ceramics such as alumina have a high sintering temperature, e.g., about 1500.degree. C., and thus they require high melting refractory metal powders of molybdenum or tungsten to make the circuit patterns. More recently, low firing temperature glasses and glass and ceramic mixtures have been employed, which glasses sinter at fairly low temperatures, e.g., below about 1000.degree. C. These glass and glass/ceramic mixtures permit the use of relatively low melting temperature metals that are more conductive than refractory metals, such as silver, gold, copper, their mixtures and alloys and the like. These low temperature ceramic substrates can be chosen to have a thermal coefficient of expansion closely matched to that of silicon, for example, and thus they have found use in circuit boards wherein silicon devices are directly bonded to the circuit boards using low melting solders or other adhesives.
Crystallizable glasses of the magnesium-aluminosilicate and lithium-aluminosilicate type have been used to make such low temperature co-fired ceramic substrates with thick film wiring patterns of conductive metals such as silver, gold or copper. Glass-ceramic insulator substrates have a low dielectric constant, which decreases the signal propagation delay in high speed digital computers, have a low resistivity to the metal conductors, and a close coefficient of thermal expansion (CTE) match to silicon which increases the reliability of solder interconnections. However, these glass/glass ceramic substrates are not as strong as alumina, and their thermal conductivity is significantly lower than that of alumina.
Another disadvantage for both alumina and glass or glass/ceramic substrates is that they shrink during firing, in all directions, which leads to problems of distortion of the layers and consequent distortion of the circuit patterns.
To overcome problems of low strength, laminated green tape stacks have been fired on prepared metal plates. These metal plates preferably contain a mechanically strong core material, such as molybdenum, tungsten, Kovar, Invar and the like, which can be plated or laminated with a layer of highly conductive metal such as copper to provide enhanced thermal conductivity. The green tape layers are stacked onto the metal plate and fired, whereupon the glass layers adhere to the metal plate. This suppresses shrinkage at least in the lateral x and y directions, with the result that all of the shrinkage occurs only in the thickness, or z, direction. This elimination of lateral shrinkage prevents distortion, warpage, and dimensional problems that adversely affect the yield of good devices. The metal plate or support substrate provides both mechanical strength and heat sinking capabilities for the ceramic multilayer circuit boards. In using this technique however, it is imperative that the coefficient of thermal expansion of the glass-ceramics be matched to that of the chosen support substrate to prevent cambering or cracking of the resulting composite substrate.
Suitable materials for fabricating low temperature ceramic substrates, particularly metal supported ceramic circuit boards, include cryatallizable glasses or mixtures of glass and ceramic capable of being sintered at temperatures below 1000.degree. C. The initial glass composition is chosen so that it undergoes complete densification and crystallization on firing to yield glass-ceramics of the required thermal, electrical and mechanical properties. The crystallization behavior of these glasses is dependent on many factors, such as their composition, their thermal history and the particle size of the starting glass powder. When mixtures of glass and ceramic are used, the softening of the glass phase at elevated temperatures leads to densification with little or no crystallization. Here the properties of the resulting ceramic can be predicted from those of the starting materials and their known proportions in the ceramic.
Up till now, the primary factors governing the choice of the dielectric composition of low temperature ceramic substrates have been the need for a low dielectric constant, which reduces the signal propagation delays in high speed digital applications, and the need for closely matching the coefficients of thermal expansion of the ceramic substrate with silicon; this enhances the reliability of direct solder interconnections between a silicon integrated circuit chip and the ceramic substrate.
Crystallizable glasses in the magnesium-aluminosilicate system, particularly those glasses having a cordierite crystalline phase, have been chosen in the past because of the known low CTE of the cordierite crystalline phase and its low dielectric constant. Stoichiometric cordierite compositions, however, do not sinter well at temperatures below 1000.degree. C. Also, they possess an unacceptably low coefficient of thermal expansion, in the range of 7-10.times.10-7/.degree. C.
To improve the sinterability and to increase the CTE of the resulting glass-ceramics, compositions rich in magnesia content, but still lying entirely in the cordierite crystalline phase, were selected by Kumar at al, see U.S. Pat. No. 4,301,324. These compositions were formulated to yield substrates having a CTE in the range of 20-40.times.10-7/.degree. C., bracketing the CTE of silicon.
Kondo et al, "Low Firing Temperature Ceramic Material for Multilayer Substrates", Multilayer Ceramic Devices, Advances in Ceramics, Vol. 19, have taught modified cordierite glass compositions containing additions of zinc oxide to improve sinterability and to increase the CTE to 24.times.10-7/.degree. C., still matching that of silicon. These compositions either lie entirely in the cordierite crystalline phase field or in the mullite crystalline phase field of the magnesium oxide-aluminosilicate ternary phase system. Holleran et al, "Glass Ceramics for Electronic Packaging", European Patent Application No. 0 289 222 A1 (1988) added certain alkali and alkaline earth oxides to magnesium oxide-aluminosilicate cordierite compositions to achieve the same result.
The predominant crystalline phase in the cordierite glass-ceramics of the prior art have been determined to be alpha cordierite, with enstatite, MgSiO.sub.3, as a secondary phase. Minor crystalline phases formed from the other additives to the glass compositions, and the residual glass, make up the glass-ceramic structure. FIG. 1 is a phase diagram of the ternary MgO--Al.sub.2 O.sub.3 --SiO.sub.2 system illustrating various possible glasses and their crystalline phase fields. The cordierite-type glasses are marked as "A".
While the above prior art compositions are suitable for fabricating free standing, co-fired, multilayer substrates, they cannot be employed with known support substrates which can be made of Kovar, Invar and the like, or composites such as of copper-molybdenum-copper, copper-tungsten-copper, copper/Kovar/copper, copper/Invar/copper and the like, or support substrates of ceramic materials such as aluminum nitride, silicon carbide and the like, all of which support substrates have a CTE in the range of 30-65.times.10.sup.-7 /.degree. C.
It would be desirable to develop glass compositions that would be suitable for fabrication of such composite structures. The dielectric glass-ceramic must adhere well to the chosen support plate, and to the thick film conductors used to form the circuit patterns and via interconnections between the circuits.
Another goal of this invention is to fabricate ceramic substrate structures having a CTE matched to gallium arsenide (GaAs) devices. Such devices are widely used for microwave applications. These glass-ceramic substrates are required to have a low dielectric constant, and very low dielectric losses in the microwave frequency range. Suitably dielectric constant is in the range of 5-7. The dielectric loss, characterized as tan .differential., should be less than or equal to 2.times.10.sup.-3.
Thus it would be highly desirable to obtain dielectric materials that would be suitable as insulators for conductors carrying high frequency digital or microwave signals having low dielectric loss factors and low dielectric constant, and also having a thermal coefficient of expansion that is compatible with metal substrates, particularly the copper coated composite substrates described above with ceramic substrates, and with gallium arsenide, which is widely used to make microwave devices. The prior art magnesium-aluminosilicate, cordierite-based glasses have some properties that are of interest, e.g., their ability to sinter to form pore-free material, their low sintering temperatures, their high rupture strength, their good resistance to chemicals used in plating, and their superior surface finish. However, cordierite glasses do not have a CTE compatible with metal or ceramic support substrates, nor to gallium arsenide.